I'm not an animator so I can't really answer this question from a creative perspective, but technically it doesn't matter where the joints are. For example the head geometry is attached to the neck joint, so when it rotates the geometry will follow. Generally the animator use the minimal set of bones they need for an expressive character and that may or may not follow the human anatomy... :-)
I'm super late to this, but I didn't see anyone else answer in the bunch of pages I looked at, so here goes The 3 "stomach" joints are actually spine joints. The joints are there to make bends between his neck and pelvis if need be (in the case of the lumberjack, I'm not sure they are needed because he's not cut up to use the extra joints. I'm also not sure if the joint export they use will transfer mesh deformation to make bends - I assume they are just exporting translation and rotation). The position of each joint point does matter, as that is where the geometry (in this case flat planes with artwork mapped on) rotates from. So for lumberjack's head, you wouldn't want to place a joint in the center because his head rotates from the neck. If he was a character that had no neck (like cartman) you could possibly center the neck joint - but even cart man rotates pretty much from the bottom center.
I'm not sure if you've ever put a paper doll together, but this is the same idea. Everywhere you place a brad is an area you rotate from.